1. Field of the Invention
This invention relates to a vibration wave driven actuator for moving a vibrator relative to a member contacting the vibrator by a travelling vibration wave produced in the vibrator.
2. Description of the Related Art
Heretofore, piezoelectric elements of an annular vibration wave motor have been arranged as shown in FIG. 6. As described, for example, in Japanese Patent Laid-open Application No. 62-201072 (1987), two groups of piezoelectric elements, A and B, are arranged around the vibration wave motor. Each electrode for the two groups of piezoelectric elements is arranged at a pitch of 1/2 of the wavelength excited in an annular vibrator, and the two groups are offset with a phase shift equal to 1/2 of the pitch, that is, 1/4 of the wavelength, from each other. Between the two groups of electrodes, there are provided a sensor electrode S for detecting the vibration state of the annular vibrator, that is, an electrode for detecting a voltage produced in the piezoelectric element and the like due to the vibration of the vibrator, a common electrode C and the like. Each of phase-A and phase-B piezoelectric elements has been subjected to polarization processing, polarity of which is alternately reversed.
By applying a voltage V=V.sub.D sin .omega.t to the group of phase-A electrodes and a voltage V=V.sub.0 sin (.omega.t.+-..pi./2) to the group of phase-B electrodes, a travelling vibration wave is produced in the vibrator, and a movable body, such as paper, film, a rotor and the like, subjected to pressure contact with the vibrator is moved due to friction.
It is possible to switch the moving direction of the movable body by (+) or (-) in the above-described expression. In the case of (-), that is, when a voltage which is delayed .pi./2 (90.degree.) in phase relative to an AC voltage applied to the phase-A electrodes is applied to the phase-B electrodes, the movable body is rotateted in the clockwise (CW) direction. In the case of (+), that is, when a voltage which is advanced .pi./2 (90.degree.) in phase relative to an AC voltage applied to the phase-A electrodes is applied to the phase-B electrodes, the movable body is rotated in the counterclockwise (CCW) direction.
A phase-S signal is detected by the sensor electrode S and is a signal which has the same frequency as that of an AC voltage applied to the phase-A electrodes. According to a phase shift between the AC voltage applied to the phase-A electrodes and the phase-S signal detected by the electrode S, it is possible to know if the vibration is in a resonance state having a large amplitude, or the degree to which the vibration deviates from the resonance state. It is further possible to control the rotation speed by determining a frequency to be applied from this information.
FIG. 7 shows relationships between the frequency f of the applied voltage and the phase difference .theta..sub.A-S between the phase-A input voltage and the output voltage of the sensor S in the case of the annular vibration wave motor having the electrode arrangement in FIG. 6, where (a) is for the case of CW, and (b) is for the case of CCW.
In FIG. 6, there are provided phase-A electrodes A.sub.1, A.sub.2, A.sub.3, A.sub.4, A.sub.5, A.sub.6 and A.sub.7 for driving, and phase-B electrodes B.sub.1, B.sub.2, B.sub.3, B.sub.4, B.sub.5, B.sub.6 and B.sub.7. That is, in this conventional example, an 8-wavefront outward deflection vibration mode which produces 8 wavelengths on the annular ring is used. Hence, as seen in FIG. 7, the frequencies f.sub.8 corresponding to this mode are normal resonance frequencies. At the frequencies f.sub.8, the phase differences .theta..sub.A-S are ideally -45.degree. for CW and -135.degree. for CCW. By comparing the above-described .theta..sub.A-S 's during rotation of the vibration wave motor with these angles, that is, -45.degree. for (a) in FIG. 7 and -135.degree. for (b) in FIG. 7, it is possible to know how close the vibration state is to the resonance state.
In the above-described conventional vibration wave motor, the center of sensitivity of the sensor electrode S deviates from the desired sensing point, that is, the point at which the phase difference .theta..sub.A-S is ideal. Typically, the desired sensing position is the physical center of the sensing point, in this case the physical center of the sensor electrode S. As shown in FIG. 8, the deviation depends on polarization directions of piezoelectric elements adjacent to both sides of the sensor electrode S constituting the phase S. Hence, a difference is produced like between (a) and (b) in FIG. 7 relative to the curve between the driving frequency f and the phase difference .theta..sub.A-S (refer to the dotted line in FIG. 7).
That is, since the polarizations for the piezoelectric elements adjacent to the phase-S electrode are in directions reverse to each other, voltages reverse to each other are produced at the interface of the elements to interfere with each other even if an identical strain is given. Hence, a voltage is hardly produced. If the polarizations are in the same direction, a voltage is easily produced, since there are no voltages which interfere with each other. Accordingly, for the entire sensor electrode S, the center of sensitivity of the phase S deviates to the right in FIG. 8.
That is, there is the problem that, even if vibration on the vibrator is in a resonance state, that state is not correctly detected.
Accordingly, there is also the problem that vibration of the vibrator is not correctly detected, and a proper driving frequency, that is, a resonance frequency for making the vibrator in a resonance state, cannot be applied to each electrode.